A bit of advice to anybody that wants it. This took me a long time to figure out even with the help of a Futaba Engineer. I Have a Robbe / Futaba FC16 transmitter running PCM. I'm building a 1:50 Walrus Class submarine (with X-rudders). The first test (above water) I used the Elevon mixer in the transmitter to mix channel 1 and 2 for the rudders. As the boat evolves the APC + depth controller were installed and I needed a x-mixer to mix the seperate depth and direction signals after the APC and Depth controller.

I was very surpised to find sluggish response from the servo's and very much stuttering. The problem turned out to be an incompatibilty between the x-mixer and the receiver.

I have an 8 channel receiver (PCM) which gives 100 pulses per servo per second in stead of the normal 50 for PPM. First I thought is was the refresh rate from the receiver that was the problem so I came up with a schematic that eliminated every second pulse to the servo's. (50 pulses per second) But the problem was still there.

You don't stop playing because you get older,
you get older because you stop playing.

Finally after some calculation I came up with the solution.Every servo gets a max pulse of 2ms. 8 servos to be supplied with a pulse so 16ms per cycle. The reciever then adds 4ms of nothing and starts with a new cycle. So every servo gets a pulse every 20ms. 50 times 20ms equals one second. A refresh rate of 50 pulses would seem the maximum possible refreshrate.

So how was it possible to get a refreshrate of 100Hz. Simple; send the pulse to 2 servo's at the same time. The microcontroller based x-mixer polls channel 1 and measures the pulselength. Then it polls channel 2 and does the same. However; normally the pulses follow each other and the mixer polls them directly after each other. Now however the pulses are 22ms apart in stead of 2ms. This results in a very low amount of servop pulses to the servo and a lot of problems.

I hope this helps others working on their x-rudder subs and maybe others. I can imagine people with a twin pro sub adding a mixer between the throttle and the rudders to make sharper turns.

EJK

You don't stop playing because you get older,
you get older because you stop playing.